Yesterday I went to start the car but when I put the key in the ignition but the key wouldn't turn!
To cut a long story short I called out RAC and they said I'd been given a 95v battery but needed 110v battery! Can I just buy one and replace it? He mentioned something about reprogramming the computer to accept the battery!? It's a 2004 td6.

Your car should start quite happily with a 95Ah battery, the 110Ah version just gives you a bit more reserve in colder weather.
I think you need to put your battery on a charger and then investigate why the battery discharged.
Despite what the RAC gentleman said there is no inter-link between the battery and any ECU on the car that needs reprogramming.
Whether such a feature is present on more up to date cars, I have no idea.

My genuine Land Rover battery is 110 Ah, and I'm pretty sure thats what it says in the manual.

As above though, should still start with a 95Ah though, just not much scope for any other issues....... 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

Surprisingly the Vehicle Handbook quotes the battery size as 90Ah or 110Ah.
Personally I would have thought 2x110Ah would be more appropriate.

Is this a key not turning issue as opposed to a battery issue ? .
Experience is the only genuine knowledge, but as time passes, I have forgotten more than I can remember
Volvo V70 P2 2006 2.4 Petrol 170bhp Estate SE
MG Midget Mk1 1962

Previous: L322 Range Rover TDV8 3.6 2008; L322 Range Rover TD6 3.0 2002; P38A Range Rover V8 1999

He tried disconnecting the battery and resetting it the key still wouldn't turn. When he used a bigger battery he had in his van the key turned fine? Land rover quoted me £212 to supply and fit a new battery and they say it should be a 110v.

I'd just go with the supply bit, and fit yourself..........at that price 2011 Range Rover Vogue SE 4.4 TDV8

Previous cars :
2003 Range Rover Vogue TD6
1999 Discovery Td5 ES
1995 BMW M5 3.8 6 speed
1992 Range Rover 3.9 Efi Vogue
1992 BMW M5 3.8
1988 BMW 735i SE
1989 Ford Sierra XR4x4 2.9i
1981 Ford Fiesta Supersport

As others have said, you should be fine with a 95Ah battery (if you manage to find a 110V battery and install it, you'll be having other problems ).

Your RAC man may be getting your older L322 confused with a newer one - they profited immensely from a software update to adjust the alternator charging curve.

You can either go to the dealer and start throwing money at it or have someone who knows what he is doing check to see exactly what the prob is. If your battery is older, it may well need replacing (you don't say when you were 'given' the 95Ah battery). If the battery is relatively new, I'd look for other problems. There are many possibilities on an L322... 
RRC 2Dr, RRC 4Dr,
P38, and 2 L322s
(wife thinks I'm nuts - prob right, too)